The development of techniques associated with mobile devices and increase in demand therefor have brought about rapid increase in the demand for secondary batteries as energy sources. Among secondary batteries, much research into lithium secondary batteries with high energy density and high discharge voltage has been conducted. Lithium secondary batteries are commercially available and widely used.
Despite the outstanding performance of lithium ion batteries, a manufacturing cost per unit cell is high, there is a risk of explosion, and depletion of lithium resources is of concern. In recent years, diverse research into magnesium batteries has been performed as an alternative to lithium ion batteries.
Magnesium batteries are secondary batteries capable of magnesium ion intercalation into and deintercalation from a cathode material using magnesium metal as an anode, thereby enabling charging and discharging. Magnesium batteries theoretically have more than twice the energy density of lithium ion batteries and are less expensive and more stable in air than lithium ion batteries. Thus, magnesium secondary batteries have drawn much attention as next-generation secondary batteries. However, many difficulties have been experienced in the development of magnesium batteries including a cathode material with higher energy density than lithium ion batteries and an electrolytic solution with a wide potential range. Only a magnesium battery using Mo6S8 as a cathode material and using Mg(AlCl2BuEt)2/THF as an electrolytic solution has been reported so far.
However, such a magnesium battery needs to be considerably improved before commercial application becomes viable. For example, a cathode current collector needs to be improved. If an aluminum foil is used as a cathode current collector and a copper foil is used as an anode current collector as in lithium ion batteries, the aluminum foil may react with Mg(AlCl2BuEt)2/THF, an electrolytic solution, and the copper foil may participate in an electrochemical reaction at a voltage higher than an operating voltage of magnesium by 1.5 V or more. Owing to these problems, a cathode prepared by coating a cathode material-binder-conductive agent mixture on a current collector formed of stainless steel foil or mesh is currently used in the magnesium battery.
The stainless steel is stable in an electric potential range of the magnesium battery of 0.3 V to 1.9 V and formed as a thin foil such as aluminum foil and copper foil or a grid-shaped mesh. However, due to high surface smoothness, mechanical strength, ductility, and malleability, adhesive strength between the active material and the foil is reduced during rolling in processing of a battery.
Therefore, there is still a need to develop technology to overcome these drawbacks.